What is the advantage to have a function or procedure created inside another function or procedure?

Hello Guys,

This is an example:

procedure SumAllValue: Integer;
	   Function Sum_A_B(a, b : integer): Integer;
	   Begin 
	   End;
	Begin 

	  My code here calls the Sum_A_B(x,y)
	End;    

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Is there any advantage using that way? Or would it be better to have it in a unit library or even create it for all my unit?

Is it only concept?

Thanks
Alexandre